<style>
    * {
        margin: 0;
        padding: 0;
    }
    .box {
        width: 800px;
        height: 350px;
        margin: 100px auto;
        background: purple;
    }
    .left {
        float: left;
        width: 200px;
        height: 350px;
        background: skyblue;
    }
    .right {
        float: left;
        width: 600px;
        height: 350px;
        background: cyan;
    }
</style>

<div class="box">
    <div class="left"></div>
    <div class="right"></div>
</div>